Towards the end of the Battle Ground, Harry travels in a boat towards the Demonreach island - a place that is a supernatural prison for even godlike beings. Yet, when he discovers that his passenger is possessed by Walker, he decides to jump overboard and orders the island to push the vessel away.
Why would he do that? On Demonreach, Harry has almost unlimited power. Not only can he control the environment of the island, but he also has the help of a powerful spirit (that he calls "Alfred"), who could withstand the prolonged attack of two Fairy Queens. So wouldn't it be smarter to lure the Walker to the island and imprison both the Outsider and the human it has possessed?